This invention relates to a heat-sensitive adhesive material which is non-adhesive at ambient temperature but becomes and remains adhesive upon and after being thermally activated. The present invention is also directed to a method of preparing such a heat-sensitive adhesive material and to a method of use thereof.
Adhesive sheets and labels have been used for various applications such as for indication of price, bar codes, quality and amount of goods and for advertisement (stickers). An adhesive sheet or label generally is composed of a substrate having one side provided with an information indicating layer and the other side on which an adhesive layer and a backing paper are laminated in this order. Information is recorded on the information indicating layer by various methods such as ink jet recording, heat-sensitive recording and pressure-sensitive recording. After recording, the backing paper is peeled away from the sheet or label and the exposed adhesive layer is bonded to a material to be bonded.
The above adhesive sheet has a problem in saving of resources because the backing paper is discarded without being recycled. Further, the adhesive sheet after the release of the backing paper poses a difficulty in handling because of the tackiness of the adhesive layer. Thus, the adhesive layer is apt to be adhered to an unwanted surface to cause curl, wrinkle or breakage thereof.
To cope with the above problems, there has been proposed a liner-free adhesive material (heat-sensitive adhesive material) which includes a substrate, an information indicating layer provided on one side of the substrate and a heat-sensitive adhesive layer provided on the other side of the substrate. The heat-sensitive adhesive layer is non-adhesive at ambient temperature but is capable of exhibiting adhesiveness when heated. The adhesive layer is composed of a thermoplastic resin, a plasticizer and, optionally, an additive such as a tackiness improver and is generally formed by applying a coating liquid containing these ingredients on the substrate.
The heat-sensitive adhesive layer does not at all show tackiness at room temperature. When thermally activated, however, the adhesive layer becomes adhesive. Such adhesiveness remains for a certain period of time even when the heat is removed therefrom. Although not yet fully clarified, the mechanism of the development of adhesiveness is considered to be such that, when the heat-sensitive adhesive material is heated, the plasticizer, when it is solid, is melted into an oily state and penetrates between the molecules of the thermoplastic resin. When a liquid plasticizer is, it is confined within microcapsules. In such a case, shells of the microcapsules are thermally destroyed, when the heat-sensitive adhesive material is heated, so that the plasticizer can plasticize the thermoplastic resin.
Because of freedom of the covering paper, the heat-sensitive adhesive material is advantageous from the standpoint of saving of resources and protection of environment. In addition, the adhesive material when bonded to an unwanted surface permits release from the surface by mere heating. Thus, the heat-sensitive adhesive material is now attractive in various fields.
Published Unexamined Japanese Utility Model Application No. H01-137263, Published Unexamined Japanese Utility Model Application No. H10-35126 and Published Unexamined Japanese Patent Application No. H01-254993 disclose the use of a heating plate, hot air, infrared rays or a thermal head for thermally activating heat-sensitive adhesive materials.
Known heat-sensitive adhesive materials have a serious problem that the adhesiveness and the anti-blocking properties are still below a level required in the market.
With regard to adhesiveness, the following problem exists. Hitherto, since polyvinyl chloride films have been used for wrapping foods, backing paper-bearing adhesive labels developed for POS (point of sales) system have been those suited for bonding to polyvinyl chloride films. Because of problems of dioxins, polyolefin films are now being substituted for polyvinyl chloride films. However, the existing backing paper-bearing adhesive labels have a problem that they fail to show sufficient adhesion strength to a polyolefin film. In particular, the adhesive layer is poor in bonding to and compatibility with a polyolefin resin film and is easily delaminated therefrom. While a lot of proposals have been made to overcome the problem, no satisfactory solution has been made. This also applies to liner-free adhesive material or heat-sensitive adhesive material.
Currently, there is a demand for a heat-sensitive adhesive material which exhibits satisfactory adhesiveness at low temperatures. However, the present inventors expect that there will be a need for a heat-sensitive adhesive material which exhibits satisfactory adhesiveness not only at low temperatures but also at higher temperatures of 35-40xc2x0 C. Namely, it is expected that there will be a demand for a heat-sensitive adhesive material which can exhibits high adhesion strength in a wide temperature range and yet can keep the adhesiveness for a long time. Such a need has never been discussed, however.
With regard to anti-blocking property, the following problems exist. Blocking is a phenomenon of undesirable occurrence of tackiness when a heat-sensitive adhesive material is exposed to a temperature higher than room temperature for a long period of time. In general, a heat-sensitive adhesive material is an elongated sheet wound around a mandrel into a roll or cut stacked sheets. Thus, when blocking occurs, the heat-sensitive adhesive layer adheres to its adjacent information indicating layer, so that the heat-sensitive adhesive material is no longer usable. When blocking occurs in a heat-sensitive adhesive material mounted on a recording device, the material cannot smoothly run through the device. When an image is recorded on the information indicating layer, blocking may cause erasure of the recorded information.
A variety of proposals have been made for the purpose of preventing blocking problems. Published Examined Japanese Patent Application No. S62-21835 proposes addition of a wax having slipping property to a heat-sensitive adhesive layer. Published Unexamined Japanese Patent Application No. H02-282050 suggests incorporation of an inorganic material into a heat-sensitive adhesive layer. Published Unexamined Japanese Patent Applications No. H06-57223, No. H06-100847 and No. H06-10848 propose protecting surfaces of solid plasticize with an inorganic compound or collide particles for the prevention of softening thereof.
The incorporation of a wax is, however, insufficient to prevent blocking. Rather, the adhesion strength of the heat-sensitive adhesive layer upon thermal activation is lowered. The addition of an inorganic material is also insufficient to prevent blocking. A solid plasticizer whose surface has been protected by an inorganic compound or colloid particles causes problems because melting and diffusing of the plasticizer are slow so that the adhesiveness is not quickly developed during thermal activation or the adhesion strength of the heat-activated adhesive layer is lowered.
Blocking could be prevented when a plasticizer having a high melting point is used. In this case, however, adhesion strength is considerably lowered. Thus, the conventional measures for improving anti-blocking properties are not satisfactory.
It is, therefore, a first object of the present invention to provide a heat-sensitive adhesive material which, upon being thermally activated, exhibits adhesiveness even at a low temperature, has high adhesion strength and retains the high adhesiveness for a long period of time.
A second object of the present invention is to provide a heat-sensitive adhesive material which, upon being thermally activated, exhibits adhesiveness not only at a low temperature but also at a high temperature, namely in a wide temperature range, has high adhesion strength and retains the high adhesiveness for a long period of time.
It is a third object of the present invention to provide a heat-sensitive adhesive material which has good anti-blocking property even when exposed to a relatively high temperature.
It has been found that the use of a supercooling improving agent is effective to provide a heat-sensitive adhesive material having a heat-sensitive adhesive layer which, when thermally activated, exhibits an adhesive strength to a polyolefin resin surface of at least 1800 gf/40 mm at 0-10xc2x0 C. and which exhibits such adhesiveness for a long period of time.
Thus, it has been found that a heat-sensitive adhesive layer which comprises a thermoplastic resin, a solid plasticizer and a supercooling improving agent exhibits higher adhesiveness at low temperatures than that of known heat-sensitive adhesive layer.
It has been also found that the first object may also be accomplished by a heat-sensitive material having a heat-sensitive adhesive layer which, when thermally activated, exhibits a scratch resistance in the range of 150-500 gf at 0xc2x0 C.
The scratch resistance is measured in accordance with Japanese Industrial Standard JIS K5401 except that a ball-point pen is substituted for a pencil. The measurement is carried out under the following conditions:
Pen: ball-point pen using an alumina ball having a diameter of 0.1 mm
Load: 1.0 kg
Scratch speed: 75 mm/min
Contact angle between pen and surface to be measured (surface of an adhesive layer): 45xc2x0
It has been also found that the above-mentioned second and third objects may be accomplished when a supercooling improving agent is used in conjunction with a solid plasticizer having a high melting point. In particular, the conjoint use of a supercooling improving agent and a high melting point plasticizer is effective to provide a heat-sensitive adhesive material having a heat-sensitive layer which, when thermally activated, exhibits an adhesive strength to a polyolefin resin surface of 500-3500 gf/40 mm not only at 0-10xc2x0 C. but also at a high temperature of about 40xc2x0 C., namely in a wide temperature range. In addition, excellent anti-blocking property such that no blocking occurs when exposed to 60xc2x0 C. or more is obtainable.
It has been further found that the above second object can be accomplished by a heat-sensitive adhesive material having a heat-sensitive adhesive layer which, when thermally activated, provides a scratch resistance in the range of 150-500 gf at both 0xc2x0 C. and 20xc2x0 C. Such a heat-sensitive adhesive material exhibits an adhesive strength of 500-3500 gf/40 mm not only at 0-10xc2x0 C. but also at a high temperature of about 40xc2x0 C.
It has been further found that the above third object can be accomplished by a heat-sensitive adhesive material having a heat-sensitive adhesive layer which shows a gloss A1 during non-activated state and a gloss A2 upon being thermally activated by being contacted with a heated surface at 80xc2x0 C. for 1 second at a pressure of 2 kg/cm2, and wherein (A1-A2) is in the range of xe2x88x9220% to 20%. Such a heat-sensitive adhesive material has excellent anti-blocking property such that no blocking occurs when exposed to 60xc2x0 C. or more.